1. Field of the Invention
The present invention relates to a radio communication system, more particularly to a receiver unit having a carrier recovery circuit which recovers a leakage carrier included in a received multilevel quadrature amplitude modulation (QAM) signal and synchronously detects the leakage carrier with a reference carrier.
2. Description of the Related Art
In a radio communication system in which data is transmitted on a radio transmission line from a transmitter unit and the transmitted data is received and reproduced at a receiver unit, a carrier is digitally modulated in accordance with data to be transmitted. One of a variety of modulation methods in practical use for such a digital radio communication system is the QAM method.
In a QAM radio communication system, a carrier is modulated in amplitude in accordance with data to be transmitted. The data is thereby transmitted as one of many corresponding coded points arranged on an imaginary plane defined by an in-phase (I) axis and a quadrature (Q) axis. This enables a large amount of data to be transmitted at one time and thus significantly increases the transmission capacity of a radio transmission line.
In a receiver unit, the carrier contained in the received multilevel QAM signal is recovered and divided into two route signals forming I- and Q-channel data signals having a quadrature relationship so as to synchronously detect the quadrature multilevel QAM data signals with carriers having a quadrature relationship. Both synchronous detected data signals are amplitude-discriminated, then the transmitted digital data are demodulated. In order to ensure precise demodulation of the transmitted data signal in the receiver unit, provision of an accurate carrier for synchronous detection of the received multilevel QAM signal is essential.
In a prior art radio communication system, the carrier is suppressed in the transmission data signal. It is, however, difficult to extract the carrier in the receiver unit. A great deal of circuit elements are required for suitably recovering the carrier.
In addition, in the prior art radio communication system, the multilevel QAM signal is generally defined by the number of coded points existing on the quadrature coordinate with an I ordinate and a Q abscissa, such as 4.times.4=16, 4.times.8=32, 8.times.8=64, or more. The most frequently used multilevel QAM method in the prior art is the 16-value method. In this case, a typical 4-phase phase shift keying (PSK) method or Costas loop application method are utilized for recovering the carrier. Also, there is further provided a phase selection control process in the carrier recovery circuit for realizing the process. The prior art carrier recovery circuit, however, cannot suppress phase fluctuation, i.e., "jitter" of the carrier, which adversely affects the synchronous detection.
On the other hand, higher multilevel QAM data, for example, 64 or more, is required to improve the transmission capacity. A prior carrier recovery circuit applied to such a higher multilevel QAM system would suffer from considerably large jitter. At the same time, the receiver unit circuit would become considerably more complex.
In order to overcome the above disadvantages, we previously devised a radio communication system wherein the transmitter unit adds a DC offset to either an I-channel or Q-channel data signal to be transmitted so as to include a leakage carrier. The receiver unit receives the transmission data including the leakage carrier, extracts the leakage carrier, and controls the phase of the leakage carrier to synchronize with the phase of the reference carrier (See "Radio Communication System", U.S. Ser. No. 566,401, filed on Dec. 28, 1983 or EPC Application No. 83307949.4, filed on Dec. 23, 1983 or Korea Application No. 83-6248, filed on Dec. 28, 1983).
While this earlier invention basically solves the problems discussed above, it is preferable to further suppress the jitter of the carrier, with a simple circuit construction, to improve phase control and to facilitate carrier extraction.